Rotor assemblies generally include a stator and a rotor. Air, nitrogen or other gas may be utilized to drive the rotor to spin about an axis relative to the stator. Gas may also be utilized to create gas bearings that support the rotor axially and radially during spinning, reducing dynamic friction to almost negligible values. The gas bearings may also function to assist in stabilizing the position and motion of the rotor during spinning. Such a rotor typically has a cylindrical body elongated along the central spin axis. The stator is often coaxially positioned about the rotor such that an axially elongated annular gap exists between the outer boundary of the rotor and the inner boundary of the stator. Gas bearing orifices in the stator direct gas radially inward toward outer surface of the rotor to create a radial gas bearing in the annular gap and thereby assist in keeping the rotor centered about the spin axis during rotation and prevent contact with the stator. Gas bearing orifices may also direct gas radially inward toward one or both axial ends of the rotor to serve as axial thrust gas bearings. Turbine blades, cups or flutes may be formed at one or both axial ends of the rotor. Gas driving orifices may direct gas to impinge against these turbine elements to drive the rotation of the rotor about the spin axis.
Some rotor designs provide turbine drive functionality at both axial ends of the rotor. In some of these rotor designs, the rotor includes axial end caps with turbine blades or flutes circumferentially spaced around outer cylindrical walls of the end caps. Multiple gas jets direct gas radially inward toward the blades or flutes in the manner of an impeller. The rotor in some of these designs employs conical axial end caps instead of cylindrical end caps with the flutes being formed in the conical surfaces. The stator may include conical inside surfaces spaced from a conical axial end cap of the rotor, thereby providing a conical gap or cavity at the axial end to improve the stability of the rotation and maintain clearance between the rotor and the stator.
In other rotor designs, one axial end of the rotor serves as the turbine and the other axial end supports an axial thrust gas bearing or a radial (lateral or transverse) end gas bearing. The turbine end is typically cylindrical with circumferentially spaced blades or flutes. Alternatively, the turbine end of the rotor of this design may be conical with flutes or vanes with the stator having a conical surface forming a conical cavity with the rotor's turbine end. A conical turbine end may also function as an air bearing and, during spinning, may produce a Bernoulli effect that tends to pull the conical turbine end into the conical cavity. As an alternative to employing an axial thrust or radial end gas bearing at the opposing end of the rotor, the opposing end of the rotor may spin on a solid point bearing that extends from the stator (or outer housing or stationary structure) into contact with the axial center of this opposing rotor end.
Rotors as summarized above have been utilized in nuclear magnetic resonance (NMR) spectrometry. An NMR spectrometer typically includes radiofrequency (RF) transmitting/receiving electronics, a sample probe, and a source of a strong magnetic field in which the sample probe is immersed such as a superconducting magnet. The sample probe contains a liquid or solid sample and one or more RF coils that serve as the electromagnetic coupling between the RF electronics and the sample. The RF electronics are operated to irradiate the sample with RF energy and receive RF signals emitted from the sample in response to the RF input. The response signals are utilized to extract information regarding the sample. Stationary (non-spinning) sample probes are adequate for many types of liquid-phase samples, but usually do not produce sufficient resolution for solid-phase samples and certain types of liquid-phase samples such as inhomogeneous liquid samples. Accordingly, rotors have been utilized to contain and spin solid and liquid samples to improve resolution in NMR techniques. The spin rate may be on the order of 106 revolutions per minute (RPM). In magic-angle spinning (MAS) techniques, the rotor is positioned to spin at the “magic angle” of 54.7° relative to the direction of the externally applied static magnetic field to further improve resolution.
Rotors employed in applications such as NMR must operate with a high level of precision and stability. Here, stability refers to restraining motion of the rotor along its axis of rotation as well as motion radial or transverse to the axis of rotation. For example, in the case of MAS Gradient systems, axial movement of the rotor on the order of 1/10,000th to 1/100,000th of an inch during gradient-refocused experiments will lead to artifacts in the detected NMR spectrum. Conventional rotor-stator systems generally do not provide the desired level of precision and stability for such NMR applications. Conventional rotor-stator systems require a multitude of drive jets impinging on the blades or flutes of a turbine to provide the rotational force needed to spin the rotor at the desired speeds. These types of rotor-stator systems impart at least two undesirable, destabilizing forces to the rotor. The first, an axial rotating net force, drives swirling and pivoting motions of the rotor that push the rotor off-axis. The second is a cogging force that occurs as each blade or flute passes over a drive jet. Cogging may also contribute to swirling and add pulsing accelerations to the rotor's motion that are also destabilizing. Destabilizing forces may be reduced or balanced by selecting certain ratios of number of drive jets to number of drive flutes, but an ideal ratio is often not practical given the spatial and geometric constraints of the typical rotor drive system.
FIG. 1 schematically illustrates an ideal rotor 100. As it spins about its rotor axis 104, the ideal rotor 100 remains perfectly centered about the rotor axis 104 along the entire length of the ideal rotor 100. While it is being driven to spin, the ideal rotor 100 does not translate axially along the rotor axis 104, nor does it translate radially relative to the rotor axis 104. By comparison, an actual, conventional rotor 108 is also illustrated in FIG. 1. In response to being driven to spin about the intended rotor axis 104, the actual rotor 108 is observed to swirl relative to the rotor axis 104 according to a rotational motion generally indicated by arrows in FIG. 1. The actual rotor 108 is also axially translated relative to its intended position represented by the ideal rotor 100. It will be understood that the deviations of the actual rotor 108 from the ideal rotor 100 are exaggerated for illustrative purposes.
FIG. 2 is a schematic elevation view of an actual rotor 208 pivoting, wobbling or precessing at an angle relative to an intended rotor axis 204 according to a pivoting motion generally indicated by arrows. The actual rotor 208 is also depicted at another point in time 212. These deviations are again exaggerated for illustrative purposes. During spinning, the same rotor 208 may undergo both the swirling motions depicted in FIG. 1 and the pivoting motions depicted in FIG. 2.
Accordingly, there is an acknowledged ongoing need for improvements in the technology of gas-driven rotors and associated systems and methods for driving and supporting rotors. In particular, there is a need for improving the stability of such rotors, and more particularly axial stability during driven rotation, including eliminating or at least substantially reducing the occurrence of destabilizing forces during the operation of such rotors.